Hot-rolled metal strip emerges from the last stand of a hot-rolling line at a very high temperature, typically about 1000.degree. C. It is therefore necessary to cool this strip to at least about 500.degree. C. both to impart the desired crystalline structure to it, to prevent the buildup of scale, and to make it possible to handle the material.
Accordingly the standard procedure is simply to pull it through a succession of high-pressure sprays that project room-temperature water on the hot strip. Due to the relatively poor heat exchange between such sprays and the workpiece, same must travel between about 60 m and 150 m to cool to the desired temperature of 450.degree. C.-550.degree. C. Once at this temperature, which is still quite hot, the strip is rolled up and set aside.
When the rolls are relatively large, 50 t being a not uncommon size, it takes a full day for them to cool by simple heat exchange with the surroundings to a termperature of 60.degree. C.-80.degree. C. at which they can be handled. While still this hot damage to the coils is fairly common so that subsequent pickling can be complicated also.
Working with these hot coils is extremely difficult and hazardous, entailing very high labor costs. The hot coils also are very hard on the equipment, requiring frequent replacement or heavy shielding and insulation of all parts.
Finally when sprays are used the underside of the strip is typically cooled much less than its upper side, so that the resultant workpiece is not of uniform crystalline structure nd granularity. The result of this is a workpiece that is wavy rather than flat.